This invention relates to an air-fuel ratio control method of controlling the air-fuel ratio of a mixture being supplied to internal combustion engines, and more particularly to a method of controlling the air-fuel ratio during a high load operating condition of the engine in which the throttle valve is substantially fully open.
A method of controlling the air-fuel ratio of the mixture being supplied to the engine during a high load operating condition is known e.g. from Japanese Patent Publication (Kokoku) No. 62-29622 by the assignee of the present application, in which the intake pipe absolute pressure and the throttle valve opening are sensed, and if both the sensed absolute pressure and throttle valve opening are below respective predetermined values, the mixture is not enriched, whereas if one of the sensed absolute pressure and throttle valve opening exceeds the predetermined value, the mixture is enriched.
Recently, a gasoline called "super A gasoline" has been sold on the market, which has low volatility as compared with an ordinary gasoline such as a C gasoline. If such a gasoline having low volatility is used, it is not well atomized so that the air-fuel ratio of the mixture becomes leaner than the case when the same amount of the ordinary gasoline is used.
According to the above-mentioned conventional method, if such a low volatility gasoline is used, even when fuel to be supplied is increased so as to enrich the air-fuel ratio of the mixture during a high load operating condition of the engine in which the throttle valve is substantially fully open, a desired air-fuel ratio, e.g. 12, and hence desired combustion cannot be obtained due to the above-described tendency of leaning of the mixture, resulting in a reduction in the engine output and hence degraded driveability of the engine. This disadvantage is conspicuous especially when the engine is in an early stage of the high load condition with the throttle valve being substantially fully open, in which the degree of volatility and hence degree of atomization of the gasoline used more largely affects the air-fuel ratio because the flow speed of intake air is higher than that of the fuel.